The present invention relates to a heat exchanger incorporating an encapsulated phase change material to provide heat storage and heat exchange. More particularly, the present invention relates to an improved phase change material heat exchanger in which the heat exchange fluid is directed along a flow path designed to induce convective currents in the molten phase change material and direct such currents to contact the remaining solid phase change material to promote solid to liquid phase change, while also being capable of promoting liquid to solid phase change when the flow direction of the heat exchange fluid is reversed.
Inorganic phase change materials (PCM's), alone or combined in eutectic mixtures, release or store heat isothermally as they change phase between liquid and solid. Phase change materials typically have high latent heats of fusion, such that significant amounts of energy can be stored in such materials as they change phase from solid to liquid, and such energy can be retrieved and dissipated to a heat sink or the like by causing the phase change material to solidify.
Latent heat thermal energy is stored in hydrated inorganic salt crystals due to the vibrating energy developed at the crystal frequency when the crystals are excited by temperature increase. At a tuned wavelength operating at a very narrow window of temperatures, salt crystals will form in a supersaturated solution. It is this property that sustains the isothermal responses. For example, it takes 72 BTU's of energy at a temperature of 242.degree. F. to vibrate and dissociate one pound of MgCl.sub.2.6H.sub.2 O crystals. Likewise, at the same frequency, the hydrate will crystallize, releasing energy isothermally at 242.degree. F. at the rate of 72 BTU's per pound of MgCl.sub.2.6H.sub.2 O.
Various hydrated salt compositions have long been recognized as candidates for use as phase change materials in thermal energy storage devices. Such compositions are listed and described in G. A. Lane, "Solar Heat Storage: Latent Heat Materials," CRC Press (1983). Because some of the hydrated salt compositions are prone to supercooling, they are typically mixed with nucleating agents.
According to the present invention, a phase change material heat exchanger is provided which comprises a container holding a phase change material, a tube surrounding the container to define an annular space therebetween, and at least two divider walls extending between the tube and the container across the annular space to divide the annular space into at least a lower flow passageway for heat exchange fluid and an upper flow passageway for heat exchange fluid.
The phase change material is selected to have a solid density greater than its liquid density. The tube is connected in fluid communication with a source of heat exchange fluid to allow heat exchange fluid to flow through the annular space to exchange heat with the phase change material. The lower flow passageway is designed to receive heat exchange fluid from the fluid source at a temperature sufficient to initiate melting of the solid phase change material, and the upper flow passageway is designed to receive the heat exchange fluid from the lower flow passageway to flow in counterflow relationship with the heat exchange fluid flowing in the lower passageway. This results in the establishment of a vertical temperature profile in the phase change material. That is, the phase change material is melted starting at a lower portion thereof to cause newly-formed liquid phase change material to be displaced to the upper portion of the container, enhancing convective heat transfer.
Advantageously, the upper flow passageway may be configured to receive heat exchange fluid from a second source at a temperature sufficient to initiate freezing of the phase change material. The phase change material is frozen starting at its upper portion so that the newly-formed solid phase change material falls by gravity to the lower portion of the container. Thus, the phase change material heat exchanger operates to effectively "store" latent heat of fusion during the heating cycle and to "release" latent heat of fusion during the cooling cycle, maintaining a vertical temperature profile during both heating and cooling cycles to take advantage of convective heat transfer.
In one aspect of the invention, the phase change material heat exchanger includes four divider walls extending between the tube and the container across the annular space. The divider walls cooperate to define a lower flow passageway for receiving heat exchange fluid from the fluid source, a pair of intermediate flow passageways for receiving heat exchange fluid from the lower flow passageway, and an upper flow passageway for receiving heat exchange fluid from the intermediate passageways. Thus, through use of this configuration, a three-pass heat exchanger is provided.
According to another aspect of the invention, the phase change material heat exchanger further includes a second tube extending through the container to provide a flow passageway for heat exchange fluid which has been discharged from the upper flow passageway. The second tube may be positioned concentrically within the container and receives heat exchange fluid for flow in counterflow relationship with the heat exchange fluid flowing in the upper passageway.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.